1. Field of the Invention
The present invention relates to devices for coating and drying a coating solution and methods thereof.
2. Description Related to the Prior Art
There are displaying devices, such as cathode-ray tube display device (CRT), a plasma display panel (PDP) and a liquid crystal displaying device (LCD).
In order to improve an angle of field in the liquid crystal displaying device, there is an optical compensation sheet between a pair of deflection plates and a liquid crystal cell. The optical compensation sheet is produced in a method disclosed in Japanese Patent Laid-Open Publication No. H9-73081. In the method, a solution containing resins is supplied on a transparent film for forming an orientation layer. Thereafter, the solution is dried and fed into a rubbing processing device for making an orientation and a coating device for coating a web with a coating solution containing liquid crystal discotic compounds on a wire bar.
As shown in FIG. 11, a conventional coating device 180 of a wire bar type includes a coat head 182 and a solution receiver 184. The coat head 182 is provided with a wire bar 181, and constructs a part of first and second manifolds 185 and 186 for providing a coating solution 183.
The coating device 180 coats a sequentially moving web 188 with the coating solution 183 by contacting the web 188 to the wire bar 181. An excess part of the coating solution 183 is received by the solution receiver 184. To the solution receiver 184, a tube 187 is attached to feed out the excess part of the coating solution 183 from the coating device 180 to a recycling device (not shown). Then, after adjusting a viscosity of the excess part of the coating solution 183, the excess part is supplied in the first and second manifolds 185, 186.
However, when a coating speed of the coating device is increased, whirls are regularly generated in the coating solution, which make wrinkles on a sheet material.
Further, in the displaying devices, a glare reflection preventing sheet is provided to prevent the decrease of the contrast and the forming of the image which are caused by reflection of the outer light.
The glare reflection preventing sheet is produced by coating a web (hereinafter web) with a coating solution and drying the coating solution in a dry air blow. Conventionally, the web is fed to a drying device by feed rollers after the web is coated with the coating solution. Thereby, a surface of the layer of the coating solution has an excess solvent. Especially, when an organic material having a low boiling point is used as a solvent of the coating solution, the solvent begins evaporating just after the web is coated with the coating solution. Further, when a long time is passed after the web is coated with the coating solution, a thermal distribution of the layer becomes larger. At a position at higher temperature, a larger amount of the solvent evaporates so that a difference of the density of the solvent in the layer becomes larger in a widthwise direction. Accordingly, a distribution of surface tension becomes large. The large distribution of surface tension causes the coating solution to flow on the web, which generates, as shown in FIG. 12, a wrinkle 191 on a surface of the glare reflection preventing sheet formed on a web 190.
In order to prevent the flow of the coating solution on the web, a dry air blow is applied to the coating solution. Further, the coating solution is condensed or a thickener is added in the coating solution to increase a viscosity of the coating solution. However, when the viscosity of the coating solution becomes larger, it is hard to coat the web with the coating solution in a high coating speed in order to form an extremely thin sheet. Accordingly, the production of the sheet material is not effectively made of the coating solution of large viscosity in the high coating speed.